The invention relates generally to fluid-filled electrical equipment. More particularly, the invention relates to an apparatus and method for determining operating status, diagnostic status, and prognostics of electrical equipment in real time.
Electrical equipment, particularly medium-voltage or high-voltage electrical equipment, requires a high degree of electrical and thermal insulation between components thereof. Accordingly, it is well known to encapsulate components of electrical equipment, such as coils of a transformer, in a containment vessel and to fill the containment vessel with a fluid. The fluid facilitates dissipation of heat generated by the components and can be circulated through a heat exchanger to efficiently lower the operating temperature of the components. The fluid also serves as electrical insulation between components or to supplement other forms of insulation disposed around the components, such as cellulose paper or other insulating materials. Any fluid having the desired electrical and thermal properties can be used. Typically, electrical equipment is filled with an oil, such as castor oil, mineral oil, or vegetable oil, or a synthetic xe2x80x9coilxe2x80x9d, such as chlorinated diphenyl, silicone, or sulfur hexaflouride.
Often electrical equipment is used in a mission critical environment in which failure can be very expensive, or even catastrophic, because of a loss of electric power to critical systems. Also, failure of electrical equipment ordinarily results in a great deal of damage to the equipment itself and surrounding equipment thus requiring replacement of expensive equipment. Further, such failure can cause injury to personnel due to electric shock, fire, or explosion. Therefore, it is desirable to monitor the status of electrical equipment to predict potential failure of the equipment through detection of incipient faults and to take remedial action through repair, replacement, or adjustment of operating conditions of the equipment. However, the performance and behavior of fluid-filled electrical equipment inherently degrades over time. Faults and incipient faults should be distinguished from normal and acceptable degradation.
A known method of monitoring the status of fluid-filled electrical equipment is to monitor various parameters of the fluid. For example, the temperature of the fluid and the total combustible gas (TCG) in the fluid is known to be indicative of the operating state of fluid-filled electrical equipment. Therefore, monitoring these parameters of the fluid can provide an indication of any incipient faults in the equipment. For example, it has been found that carbon monoxide and carbon dioxide increase in concentration with thermal aging and degradation of cellulosic insulation in electrical equipment. Hydrogen and various hydrocarbons (and derivatives thereof such as acetylene and ethylene) increase in concentration due to hot spots caused by circulating currents and dielectric breakdown such as corona and arcing. Concentrations of oxygen and nitrogen indicate the quality of the gas pressurizing system employed in large equipment, such as transformers. Accordingly xe2x80x9cdissolved gas analysisxe2x80x9d (DGA) has become a well accepted method of discerning incipient faults in fluid-filled electric equipment.
In conventional DGA methods, an amount of fluid is removed from the containment vessel of the equipment through a drain valve. The removed fluid is then subjected to testing for dissolved gas in a lab or by equipment in the field. This method of testing is referred to herein as xe2x80x9coff-linexe2x80x9d DGA. Since the gases are generated by various known faults, such as degradation of insulation material or other portions of electric components in the equipment, turn-to-turn shorts in coils, overloading, loose connections, or the like, various diagnostic theories have been developed for correlating the quantities of various gases in fluid with particular faults in electrical equipment in which the fluid is contained.
However, since conventional methods of off-line DGA require removal of fluid from the electric equipment, these methods do not, 1) yield localized position information relating to any fault in the equipment, 2) account for spatial variations of gases in the equipment, and 3) provide real time data relating to faults. If analysis is conducted off site, results may not be obtained for several hours. Incipient faults may develop into failure of the equipment over such a period of time.
MICROMONITORS, INC(trademark) and SYPROTEC(trademark) have each developed a gas sensor which resides in the drain valve, or other single locations, of a transformer and overcomes some of the limitations of off-line DGA. However, location data relating to a fault is not discernable with such a device because it is located in one predefined position and does not provide any indication of the position of the source of the gas, i.e., the fault. U.S. Pat. No. 4,654,806 discloses an apparatus for monitoring transformers including sensors for detecting oil temperature, gas in oil, and cabinet temperature. Raw data from the sensors is collected by a microcomputer and periodically downloaded to remote host computer. The microcomputer can compare various measured parameters with predetermined thresholds and can activate alarms or other warnings if the thresholds are exceeded. The remote host computer can control a cooling system of the transformer based on the parameters that are periodically downloaded to the remote host computer. Similarly, U.S. Pat. No. 3,855,503 discloses a remotely monitored transformer in which data from sensors is downloaded to a remote computer and compared to predetermined thresholds. If the thresholds are exceeded, the transformer can be de-energized. U.S. Pat. No. 4,654,806 discloses that the individual thresholds can be varied based on other thresholds. However, the devices disclosed in U.S. Pat. No. 4,654,806 and U.S. Pat. No. 3,855,503 fall short of providing comprehensive and cohesive diagnostics in real time because they do not account for the complex relationships between the various operating parameters of fluid-filled electrical equipment or the normal degradation over time of fluid-filled electrical equipment. The article entitled xe2x80x9cMonitoring the Health of Power Transformerxe2x80x9d discusses research at Massachusetts Institute of Technology relating to model based diagnostic systems.
Known processes and apparatus do not provide accurate, real-time diagnosis of incipient faults in, and prognosis of, fluid-filled electrical equipment because the complex relationship between various operational parameters of fluid-filled electrical equipment is not addressed fully by the prior art. For example, a temperature rise outside of a normal range may be due to a temporary increase in load and not to an incipient fault. Other parameters are related in more complex ways that are not addressed by the prior art. Also, the devices discussed above do not account for dynamic change over time in transformer behavior.
U.S. Pat. No. 5,845,272 discloses a system for isolating failures in a locomotive or a process having a plurality of equipment. The system uses outputs from various sensors as inputs in a knowledge base including causal networks. However, U.S. Pat. No. 5,845,272 is not directed to diagnostics of fluid filled electrical equipment and thus does not take into account the complex relationships between parameters of fluid filled electrical equipment and the dynamic change in behavior of fluid-filled electrical equipment over time.
In summary, known processes and apparatus do not take into account analytical models of fluid-filled electrical equipment operation including thermal, fluid flow, electric field, pressure-volume, chemical, failure mode, root failure cause and gas-in-oil models, all of which are related in a complex manner and change over time. Therefore, known methods and apparatus do not accurately identify and predict failure modes and assess the life cycle of fluid-filled electrical equipment.
The invention relates to a diagnostic apparatus and method for electrical equipment. A first aspect of the invention is an intelligent analysis apparatus for fluid-filled electrical equipment of the type having components surrounded by fluid. The apparatus comprises plural sensors adapted to be coupled to fluid-filled electrical equipment and configured to output signals indicative of plural operating parameters of electrical equipment, and a diagnostic device coupled to the sensors and having a processor for determining operating characteristics of the electrical equipment based on at least one analytical model of the electrical equipment and the signals outputted by the sensors by applying values of parameters calculated by the at least one analytical model and values of parameters as indicated by the signals of the sensors in a causal network.
A second aspect of the invention is a method for intelligent analysis of fluid-filled electrical equipment of the type having components surrounded by fluid. The method comprises the steps of sensing plural operating parameters of electrical equipment having a containment vessel configured to contain a fluid and at least one electrical component disposed in the containment vessel, generating signals indicative of the plural operating parameters of the electrical equipment sensed in the sensing step, and determining operating characteristics of the electrical equipment based on at least one analytical model of the electrical equipment and the signals generated in the generating step by applying values of parameters calculated by the at least one analytical model and values of parameters as indicated by the signals generated in the generating step in a causal network.